The present invention relates to an intake air amount detecting system employing a temperature-dependent resistor which is electrically heated.
In a system disclosed in U.S. Pat. No. 3,747,577, a temperature-dependent resistor is disposed in an intake air path of the engine, and is connected as one arm of a bridge circuit. A diagonal point of the bridge circuit is connected to the input terminal of an amplifier, and the electric current for heating the resistor is controlled by the output of the amplifier. Namely, the resistor also serves as a sensor for detecting its own temperature. The change in the temperature of the resistor caused by the change in the air flow velocity is detected in the form of change in balance of the bridge circuit. Therefore, the heating circuit is controlled, and the temperature of the resistor is adjusted to remain constant. With this arrangement, the output of the amplifier is used as a signal which represents the intake air flow rate to control the fuel injection of the engine.
In the intake air amount detecting system of the type mentioned above, the response characteristics for producing the detection signal that represents the intake air flow rate vary depending upon the thermal capacity of the sensor unit formed by the temperature-dependent resistor. However, if the mechanical strength and durability of the sensor are taken into consideration, there is a limit to which the thermal capacity of the sensor can be reduced. Especially, if the sensor is constructed to be very durable so that the resistor will not be damaged even when the engine develops a back-fire, the thermal capacity of the sensor becomes so great as to create a problem with regard to the response characteristics of the sensor.